From an evolutionary standpoint we're hard wired to be happy. Happiness can be synthesized.

There's something known as the "Impact Bias" which states that any given situation may end well or poorly will have far less impact and duration on us than we really expect them to have. 

If something happened to you 3 months ago, like say the loss of an important person, it will have very little impact on you with few exceptions. You will mourn and be sad, but you also will learn to find a new normal. Not to say you will forget about said person, but all things considered you will be by all accounts doing okay. Sadness isn't bad it simply is one emotion.

Happiness is another emotion and the human mind is not built to only sustain one type of emotion. Our emotions act as a compass, imagine if the compass you used only pointed north. You'd get nowhere. 

What we worry and stress out about is overstated.

When we choose to reframe experiences, it's something that we need to consistently be working on if we want to be happy. It's not hard but it is something you need to cultivate as is any other habit or skill. Happiness isn't so much a choice as it is a conscious decision of being present; it may not make you instantly happier but over time you will raise your overall level of happiness.